This disclosure relates to process control, and, in particular to process control for fluids.
Fluid regulators (e.g., valves) are used in a wide variety of environments for both commercial and industrial applications. For example, fluid regulators can be used to regulate fluids in pipeline systems, chemical plants, and building environmental systems. Regardless of environment or application, many fluid regulators are controlled by some type of logic-driven controller, whether located with or remote from the fluid regulator. A controller allows a fluid regulator to be operated in an intelligent manner to achieve one or more defined performance measures (e.g., flow rate, pressure, temperature, level, energy efficiency, etc.).
A common type of controller operates in response to pneumatic control techniques. This type of controller often receives an external air supply and manipulates the air supply in response to electronic control signals to actuate the fluid regulator appropriately. Thus, a pneumatic controller is often said to include an electric-to-pressure converter that converts an electronic control signal to a corresponding pneumatic control pressure. Depending on the fluid regulator to be actuated, a pneumatic fluid regulator controller may also include a second pneumatic component. This second stage of the controller may amplify the pressure and/or volumetric flow rate of the output of the electric-to-pressure converter. Common devices for accomplishing this include a spool valve and a pneumatic relay.
The flow rate of a fluid through a pneumatically-controlled fluid regulator depends in part on the pneumatic control pressure, which, in turn, depends on the electronic control signal received by the electric-to-pressure converter. The relationship between the flow rate of the fluid regulator and the electronic control signal is often non-linear, such that a particular increase in control signal does not correspond to a first order increase in the flow rate of the fluid regulator.